Personnel lifts are used for a wide variety of applications. A typical personnel lift includes a work platform that can be raised or lowered to position a worker at a desired height. The work platform and the worker can be raised to a position where the worker can paint overhead surfaces, trim tree branches, or work on overhead fixtures, for example.
Recently, personnel lifts have become a popular rental item. Rental provides a relatively inexpensive way for an individual or company to use a personnel lift for a short period of time. The user does not have to store the personnel lift, and is not responsible for periodic maintenance of the personnel lift.
Personnel lifts can be bulky and large, and transporting a rented personnel lift to a work site may be difficult. Often, with larger personnel lifts, the rental of a truck or other transportation vehicle to move the rented personnel lift to a work site may exceed the cost of rental of the personnel lift.
To aid in mobility, and decrease the cost thereof, manufacturers have recently started providing personnel lifts on trailers. For ease of reference, the trailer-mounted personnel lifts will hereinafter be referred to as "trailer personnel lifts." A trailer personnel lift may be towed behind a vehicle with a conventional trailer hitch. Once the trailer personnel lift is towed to the work site, the personnel lift is ready for stabilization, leveling, and use.
A trailer personnel lift typically employs four outriggers at the right front, left front, right rear, and left rear of the device for stabilizing the trailer personnel lift. On most prior art trailer personnel lifts, outriggers are manually lowered to stabilize the personnel lift. A simple tilt sensor, such as a pendulum-based electronic sensor, is used to determine whether the trailer is level and provide a lockout that prevents the operation of the personnel lift until the trailer is level. The pendulum-based electronic sensor consists of a disk that is suspended by a cable into a vertically oriented cylinder. If the disk contacts one side of the cylinder, the sensor indicates that the trailer is not "level". The pendulum-based sensor, however, does not indicate the direction in which the trailer is leaning. Instead, leveling bubbles are provided between the outriggers that indicate the direction of trailer tilt. Using the leveling bubbles and the pendulum-based electronic sensor, workers adjust the outriggers on the trailer until the trailer is level.
There are several problems with the leveling system that utilizes a pendulum-based electric sensor and bubble levels. As discussed above, a pendulum-based sensor does not indicate the direction in which a trailer is leaning. Leveling a trailer may be difficult because the individual bubble levels can only indicate level along one axis. Operators often attempt to level a trailer by eye-balling two or more bubble levels. Unfortunately, bubble levels are not very accurate and are often confusing to an untrained operator. In addition, "level" on the bubble levels and "level" on the tilt sensor may not correspond.
Further, as noted above, a pendulum-based electronic sensor does not indicate how level a trailer is, only that the trailer is not level. During setup, an operator can adjust the outriggers such that while the pendulum-based electronic sensor indicates that the trailer is level, the pendulum is not centered in the cylinder. Rather, the pendulum is nearer one side of cylinder than the other sides. During operation of the personnel lifts, a slight shift of the trailer may cause a pendulum near one side of the hanging cylinder to come into contact with that side. Due to its lockout function, such contact will disable the lifting system of the personnel lift. Specifically, the "up" function for the work platform will be shut down. Some models also shut down all functions, which leaves an operator stranded on the aerial work platform until a worker is available at ground level to re-level the trailer by adjusting the outriggers, or manually lower the operator by using a set of override controls located at the base.
Thus, there exists a need for a new and improved leveling system for a trailer personnel lift. The leveling system should be capable of determining how level the personnel lift is, so that slight shifts of the trailer personnel lift during operation will not cause the personnel lift to shut down.